Low melting or liquid phosphonium and imidazolium salts have found utility as polar solvents known as “ionic liquids”. Ionic liquids provide an attractive alternative to traditional organic solvents for chemical reactions for many reasons. Ionics liquids display low vapour pressure which, for industrial purposes, is a very important feature. They are essentially non-volatile, a property that eliminates many of the containment problems typically encountered with traditional organic solvents. Since ionic liquids are often composed of poorly coordinating ions, they have the potential to provide a highly polar yet poorly coordinating solvent. Moreover, many of these solvents are immiscible with traditional organic solvents and therefore provide a non-aqueous polar alternative for use in two-phase systems. Because of their distinctive solvent characteristics, they can be used to bring unusual combinations of reagents into the same phase. A recent review of the properties and uses of ionic liquids is provided in an article entitled “Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis,” by Thomas Welton (Chem. Rev. 1999, 99, 2071-2083).
Ionic liquids provide solvents with a wide liquid range and a high degree of thermal stability. However, there remains a need for increasing the solvent options available to chemists by developing novel ionic liquids with distinctive physical and chemical properties.
Ionic liquids can be prepared by a two step process, comprising the steps of (a) reacting a nitrogen-containing compound (for example, imidazole) or a phosphorus-containing compound with an alkylhalide to obtain a quaternary nitrogen or phosphorus halide salt; and (b) exchanging the halide ion with a suitable anion (ion exchange or metathesis) to obtain a low-melting quaternary nitrogen or phosphorus salt. This process has several drawbacks. For example, the end-product can be contaminated with residual halide ion, which may interfere with the activity of halide-sensitive catalysts. For instance, halide ions such as chloride ions coordinate with group VII metals such as palladium and platinum. If an ionic liquid is to be used in an environment where halide ions are unacceptable, even at low levels, halide salts should not be used in the starting materials or a further process must be used which ensures removal of halide ions from the ionic liquid. Also, the two-step process is inconvenient, as the ion-exchange step produces salt or acid side-products that must be removed by washing with water.